mat foundation reinforcing

huangyhg

mat foundation reinforcing
i'm designing a mat foundation to support material handling equipment.  the equipment is basically a short (in height) series of beams and supports that transfers long bundles of material across it.  when i normally design a mat foundation, it's for a silo or tower and ends up being rather thick, and reinforced with a top and bottom mat of steel.  this mat fdn for the mat'l handling equipment does not need to be more than 8" thick with shrinkage and temperature steel as the min. reinforcing required.  
my intent is to place the steel at mid height of the slab.  typically, i use a top and bottom mat of reinforcing designed for the positive and negative moments between supports.  but in this case, i feel that is overkill.  
anyone designed something similar that they could share their experiences on?
if you providing shrinkage reinforcement, you may place it close to top (where shrinkage will occur). i would place it 1.5-2.0 inches from the top.
good luck.
sometimes "overkill" is not bad. for slabs that may have unpredictable loading, the top and bottom rebar mats that you typically use are well suited. i prefer a minimum 12 inch thick concrete slab with #4 @ 12 inches, on center, each way, top & bottom as a minimum design for an industrial material handling mat.
consider the economics of a 12 inch slab vs and 8 inch slab:
1. excavation cost may a little higher, but not much.
2. subgrade preparation costs are identical.
3. there is "room" for meaningful top and bottom rebar mats. in the 8 inch slab the rebar is at, or near, the neutral axis and is of limited value.
4. the formwork is virtually identical.
5. concrete placement cost may be a little higher, but not much.
6. concrete finishing costs are identical.
7. concete curing costs (curing compound, etc.) and time required are identical.
8. you do buy 50% more concrete, but you are getting it installed for essentially the cost of the concrete.
to me, the extra 4 inches is cheap insurance.
also the steel will actually work in the slab.  of course the owner will never overload the slab by bringing in a load of steel beams and drop one or more.